1. Technical Field
This disclosure relates generally to optical discs and, more specifically, to photopolymer resins that can be used for photo replication of information layers of optical discs and related applications. Thus, this disclosure relates to optical disc applications such as Blu-ray (BD-RW, BD-RE) and high-density DVD (HD-DVD) discs
2. Description of the Related Art
One example of a popular optical storage device is the compact disc (CD). A CD can store large amounts of digital information (783 MB) on a very small surface that is inexpensive to manufacture. The CD surface is essentially a mirror covered with billions of tiny pits that are arranged in a long, tightly wound spiral or groove. The CD player reads the pits with a precise laser and interprets the information as bits of data.
The spiral of pits on a CD starts at the center of the disc. CD tracks are approximately 0.5 microns wide, with 1.6 microns separating one track from the next. The elongated pits are each about 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high.
Most of the mass of a CD is an injection-molded piece of clear polycarbonate plastic that is about 1.2 millimeters thick. During manufacturing, this plastic is impressed with the microscopic pits that make up the long, spiral track A thin, reflective aluminum layer is then coated on the top of the disc, covering the pits.
When a CD is played or read, a laser beam of the CD player or drive passes through the polycarbonate layer of the CD, reflects off the aluminum layer and hits an optoelectronic device that detects changes in the light. The pits reflect light differently than the flat parts of the aluminum layer, which are called lands. The optoelectronic sensor detects these changes in reflectivity, and the electronics in the CD-player drive interpret the changes as data bits.
For removable storage applications, CD-recordable (CD-R) and CD-rewritable (CD-RW) devices are used CD-R works by replacing the aluminum layer in a normal CD with an organic dye compound This compound is normally reflective, but when the laser focuses on a spot and heats it to a certain temperature, it “burns” the dye, causing it to darken. To retrieve the data written to the CD-R, the laser moves back over the disc and treats each burnt spot as a pit. Data can be written to a CD-R only once; after the dye has been burned in a spot, it cannot be changed back. CD-RW and DVD discs address this problem by using phase change, which relies on a very special mixture of antimony, indium, silver and tellurium.
To provide higher data capacity CD and DVD discs to satisfy the need for higher imaging recording, high density video and TV recording, interactive DVD movies, and game applications. To increase data capacity, shorter wavelength lasers are used.
For example, the first CD formats used 780 nm lasers, then DVD formats used 650 nm lasers, and more recently Blu-ray (BD-RW, BD-RE) or high density DVD (HD-DVD) formats use 405 nm lasers. With the availability of shorter wavelength lasers, smaller pits (or pits) and narrower, more tightly packed grooves are made on the disc and more data can be packed into a given area on the disc. To further increase data density of optical discs, multiple data information layers can be constructed on the disc. In short, the data density of the high density optical discs can be increased by two ways: 1) more pits and grooves can be packed on the surface of the optical discs due to the availability of the shorter wavelength lasers and higher NA lenses, and 2) multiple information layers can be built vertically to provide mote data layers within the optical discs.
Examples of DVD-type optical discs with single, dual and four data layers are shown in FIGS. 1A-1C respectively. A partial view of a DVD disc 10A is shown in FIG. 1 with a single information layer 12. To build the multiple information layers such as those shown at 12, 14 and 16, 18, 20, 22 for the discs shown 10B and 10C in FIGS. 1B and 1C respectively, photopolymer resins (also known as “2P resins”) are used for pit and groove replications. As shown in FIGS. 1B and 1C, the information layers ate separated from one another by an optically clear spacer layer shown at 24, 26, 28, 30.
There are several ways to produce multiple data layers on optical discs. One way for making such data layers is called the 2P process because of its use of photopolymer to emboss the pits and grooves on a data layer of an optical disc. The pits and grooves represent prerecorded digital information in the case of prerecorded media or tracking or header information in the case of recordable media. A typical or traditional 2P process is shown in the schematic flow diagram of FIG. 2.
In FIG. 2, an injection-molded first information layer is shown at 41, which has a semi-reflective layer (not shown). The first information layer 41 is coated with an optically clear resin that is eventually cured to form a first spacer layer 42. The first spacer layer 42 is then coated with a photopolymer resin 43 which, in turn, is engaged by the stamper 44. The stamper is preferably optically clear thereby enabling UV energy to pass through the stamper 44 so as to emboss data from the stamper 44 into the cured photopolymer layer 43 which is disposed on top of spacer layer 42. The stamper 44 is then removed.
In a traditional 2P process, two types of UV resins are required. First, the spacer layers 42 are made from UV resins that have good adhesion to the semi-reflective layer 41, which is made of metal or ceramic. The resin for the spacer layer 42 must have low shrinkage so that the radial and tangential tilts of the discs are low for good tracking of the optical discs at high rotational speeds. The second resin is a UV-curable resin for the layer 43 that has good adhesion to the cured spacer layer 42 and that can be separated readily from the transparent plastic stamper 44 after the UV exposure step.
In this technique, one plastic stamper 44 is used for each data layer. Various materials can be used for the stampers, including polycarbonates (PC), polymethylmethacrylates (PMMA), and polyolefins (PO) Only polyolefins have been successfully used without the aid of an external release layer on the stamper surface. However, polyolefins are expensive as is the alternative which would be an additional release layer on the stamper. Further lower cost plastic materials such as PC and PMMA ate polar, and their adhesion to 2P resin is too great for clean separations from the plastic stamper after the UV curing step.